Amazon thought that if they could force users to go straight...

Amazon thought that if they could force users to go straight to Amazon, offer a small amount of money to charity from each order, it would be a far reduced amount than they would owe Google. It therefore helps kill customers going to Google, saving Amazon more money and being positive overall for the brand value of Amazon. 

That’s why for the program to work, the user has to start shopping at Smile.amazon.com so Google doesn’t get attribution. As a matter of fact, until recently, the option to use Smile wasn’t available in the app. Amazon Smile was developed by the Traffic Optimization team, whose entire purpose is increasing efficiency and lowering costs of getting customers to Amazon.

www.joshbeckman.org/notes/462580780